Optical oximetry, which is based upon the difference in the absorption properties of oxyhemoglobin and deoxyhemoglobin, was used by Donahoe et al. to measure tissue, as well as arterial oxygen saturation. ("A New Noninvasive Backscattering Oximeter", T. M. Donahoe and R. L. Longini, Proceedings of the IEEE/Seventh Annual Conference of the Engineering in Medicine and Biology Society, pp. 144-147, 1985.)
The Donahoe et al. paper describes a noninvasive backscattering oximeter which utilizes photon diffusion theory to analyze the optics of blood in perfused tissue. In backscattering oximetry, a light source and light detector are placed side by side on the same tissue surface, whereas in transmission oximetry, two or more wavelengths of light are transmitted through the tissue and the effect on the transmitted light is measured to evaluate the degree of oxygen saturation of hemoglobin. Specifically, the Donahoe oximeter had the ability to detect changes in the volume of blood cells relative to that of tissue which the authors conveniently defined as "tissue hematocrit". Although "tissue hematocrit" and blood hematocrit may be related, the exact relationship is not known and can not be accurately defined or predicted for each individual. Furthermore, the accepted parameter which is most commonly used in clinical medicine is referred to as "blood hematocrit", which is defined as the volume percentage of erythrocytes in whole blood, and not "tissue hematocrit", as defined by Donahoe, et al. Moreover, the method described by Donahoe, et al. requires careful calibration to correct for variations in tissue scattering and absorption among individuals and among different sites, even on one individual.
The calibration method proposed by Donahoe et al. requires the application of pressure to the tissue site where the measurement is performed in order to render the tissue site bloodless. This calibration procedure results in large errors for two primary reasons: 1) it is difficult to know exactly the amount of blood in the tissue during calibration or whether the tissue was rendered completely bloodless, and 2) the application of pressure to biological tissues deforms the tissue structures which, in turn, results in different optical properties compared to that of the undeformed blood perfused tissue.
Sperinde et al., in U.S. Pat. No. 4,523,279 issued Jun. 11, 1985, disclose an invasive oximeter technique in which light at three different wavelengths is optically integrated and coupled through an optical fiber to an aperture in a distal tip of a catheter disposed within a blood vessel. Back-scattered and reflected radiation is received through a separate optical fiber to a second aperture in the tip and coupled to a central processor. Blood oxygen saturation is measured as a function of the radiation intensities from the back-scattered light at the three different wavelengths normalized with respect to a reference light intensity measurement.
Takatani et al., in U.S. Pat. No. 4,867,557 issued Sep. 19, 1989, disclose a non-invasive reflection type oximeter which requires light beams at six different wavelengths to be applied to the body tissue. Light received from the six different beams, after being absorbed in the tissue, is detected by a single detector and processed in accordance with a predetermined function to determine the quantity of hemoglobin and oxygen saturation of the body tissue. A light interception barrier separates the transmit light beams from the single light receiving element to prevent direct light cross-talk (or coupling) of light on the receiving element. This oximeter is directed to body parts which do not contain a pulsatile component and requires constant light intensity.